1. Field of the Invention
The invention relates to an exhaust gas purification catalyst, and more particularly relates to a ceramic support that is a constituent of this catalyst.
2. Description of Related Art
Three-way catalysts are widely used as catalysts for the purification of the hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxides (NOx) present in the exhaust gas emitted from engines in, e.g., automobiles and so forth. In a typical three-way catalyst structure, for example, an alumina coating layer is formed on the surface of a highly heat-resistant ceramic substrate and platinum (Pt), palladium (Pd), and rhodium (Rh), which are noble metal catalysts, are supported on this coating layer.
In order to efficiently purify the aforementioned exhaust gas components using such a three-way catalyst, i.e., in order to convert them into H2O, CO2, and N2 by oxidation or reduction, the air-fuel ratio, i.e., the mixing ratio between the air and gasoline supplied to the engine, must be near the stoichiometric air-fuel ratio (stoichiometry). With the goal of increasing the width of the catalyst purification window, i.e., the range of the air-fuel ratio in which the catalyst can effectively function, an oxygen storage material having an oxygen storage capacity (OSC), and typified by cerium oxide (CeO2), is also generally widely used in the exhaust gas purification catalyst. The oxygen storage material present in an exhaust gas purification catalyst works as follows: when the air-fuel ratio in the exhaust gas is lean (that is, an atmosphere on the excess oxygen side), it stores the oxygen in the exhaust gas, and when the air-fuel ratio in the exhaust gas is rich (that is, an atmosphere on the excess fuel side), it releases the stored oxygen. This provides a stable catalyst performance even when the oxygen concentration in the exhaust gas varies and thereby improves the purification performance of the catalyst. In an example of a typical catalyst structure that employs an oxygen storage material, a composition in which alumina and the oxygen storage material are mixed in prescribed proportions is coated on the surface of the substrate and noble metal catalyst (Pt, Pd, Rh, and so forth) is supported thereon.
In order to obtain additional improvements in the purification performance, exhaust gas purification catalysts have been proposed in the last few years in which the catalyst coating layer is executed as a two layer structure and Rh is supported separately from the Pt or Pd. Here, the entire noble metal catalyst is not supported in a single support layer; rather, the catalyst coating layer is formed as a layer structure that has at least two layers, i.e., an upper layer and a lower layer, and Pt or Pd is supported in one layer and Rh is separately supported in another layer. This has the effect of inhibiting the decline in catalytic activity caused by Rh alloying with the Pt or Pd. For example, Japanese Patent Application Publication Nos. 2009-648 (JP 2009-648 A), 2010-115591 (JP 2010-115591 A), and 2010-119994 (JP 2010-119994 A) describe exhaust gas purification catalysts that have a two layer structure formed of a lower layer and an upper layer, wherein Pd or Pt is supported in the lower layer on a support that contains a Ce—Zr composite oxide (also referred to as “CZ composite oxide” below), which is an oxygen storage material, and Rh is supported in the upper layer on a support that contains, for example, CZ composite oxide.
On the other hand, as noted above, a mixture of cerium oxide (typically CeO2), which has an OSC, and alumina (Al2O3) is in wide use as a noble metal catalyst support. However, cerium oxide has a lower heat resistance than alumina, and when used at high temperatures, the crystalline structure changes and/or crystal growth advances, resulting in a decline in the specific surface area. As a result, when a three-way catalyst that contains a noble metal catalyst and cerium oxide is used in the high temperature region of 800° C. and above, the OSC of the catalyst will be substantially reduced after this. This is accompanied by a decline in the low-temperature purification performance of the catalyst post-durability testing.
As a consequence, with the goal of inhibiting crystal growth by the cerium oxide, CZ composite oxides or solid solutions provided by the addition of zirconium oxide in addition to cerium oxide are in wide use as oxygen storage materials (for example, JP 2009-648 A). However, the low-temperature catalytic activity of the catalyst post-durability testing has still not been satisfactory even using a CZ composite oxide.
Japanese Patent Application Publication No. 10-202102 (JP 10-202102 A) thus discloses for the first time a technology that uses an oxygen storage material in the form of an aluminum (Al)-cerium (Ce)-zirconium (Zr) composite oxide produced from metal alkoxide. According to JP 10-202102 A, this Al—Ce—Zr composite oxide is formed of small primary particles provided by mixing Ce and Zr with Al to uniformity at an atomic or molecular level, and a catalyst having Pt and Rh supported on a support that contains the Al—Ce—Zr composite oxide exhibits a greater inhibition of the OSC drop post-durability testing than a catalyst having Pt and Rh supported on a support in with alumina is simply mixed with a CZ composite oxide.
Additional improvements in purification performance are desired for these exhaust gas purification catalysts. In particular, after durability testing by exposure to high-temperature exhaust gas, these catalysts exhibit a substantial drop in catalytic activity from their initial level. For a cerium oxide-containing support loaded with a noble metal catalyst, one cause of this substantial drop in catalytic activity is believed to be that the OSC of the catalyst after use in high-temperature durability testing exhibits a substantial decline from the OSC in initial use. As noted above, one reason for the drop in the OSC after durability testing is the advance in crystal growth by cerium oxide in high temperature regions.
Moreover, accompanying the strengthening of fuel economy requirements in recent years, the exhaust gas temperatures of not only diesel engines but also gasoline engines are trending downward. For example, the gasoline engine in a hybrid vehicle experiences very frequent operation under low temperature conditions. Accordingly, it has become critical that the catalytic activity also not be reduced at low temperatures. However, the previously described exhaust gas purification catalysts have not had a satisfactory low-temperature activity post-durability testing.